The present invention is generally related to automatic control systems and is more particularly directed to a system for controlling operation of a device utilizing a capacitive sensor.
A great number of capacitive sensors have been heretofore developed for the sensing of persons or materials to provide an alarm, indicating signal, or control. For example, capacitive sensing circuits have been used for alarm systems to provide a signal in response to touching of a particular area or the proximity of an object. In other instances, capacitive sensing circuits have been utilized to detect the presence or absence of liquids and solids and thereafter initiating an indicator for alarm signals or measurement. Capacitive sensors have also been used to measure the distance to an object, material size, material moisture content, oil contamination, humidity, pressure, liquid level and in fact have formed the basis for sensing in numerous measurement and detection applications.
With regard to dispenser control, it is often preferable to operate a device without direct handling thereof by human interaction. For example, it is preferable for sanitary reasons in washing to avoid the need for physical contact with faucet handles, towel dispensers, hand dryers, soap dispensers and the like.
While a number of control systems have been developed for such touch-free devices in order to conserve water and soap, they have been plagued by false activation. That is, devices are turned on without the actual presence of a human body part. This, of course, leads to fluid waste that is contrary to the original purpose of the control system.
Further, in the case of soap dispensers and the like, safety becomes a factor when such liquids are falsely dispensed and accumulate on a floor, or other surface, where subsequent slippage thereon may cause bodily harm.
The problem of false activation, and more generally of reliable as well as sensitive detection of a proximate object by a proximity sensor, stems from the need to reliably discriminate between a small change in signal strength due to changes in the proximity of the object versus changes in signal strength which can occur due to other factors such as sensor noise, sensor drift or induced changes in the signal due to actual changes in the ambient environment itself, such as contamination of the sensor and other effects which can give rise to signals which are similar in magnitude to or even larger than the detection signal itself.
In the case of infrared proximity sensors, which are for instance frequently used in current commercial non-contact soap dispensers and other similar devices, false activation can arise due to the effects of stray, extraneous light impinging on the sensor due to spurious reflections in shiny objects or otherwise, or a failure to detect an object can occur due to variations in the reflectivity of the object or contamination of the optics.
In the case of capacitive proximity sensors, where an object is sensed via the detection of a change in capacitance due to the proximate presence of the object, sensitive detection of a proximate object in everyday environments is made difficult and unreliable because the actual capacitance changes due to a proximate object can be small compared with other changes in capacitance due to changes in the surroundings.
Certain commonly occurring variations in the environment which can cause such interfering variations in capacitance include contamination of the surface of the electrodes or other structures in the sensing field region by gradual dirt accumulation or condensed moisture, significant changes in ambient humidity, gradual variations in the proximity or composition of other nearby structures and objects, or variations in sensor mounting location, all of which can give rise to small alterations in the electric field shape or intensity between the sensor electrodes thereby altering the charge state and hence capacitance between the electrodes.
There are currently two basic types of capacitive proximity sensor in the known prior art. In one case, often referred to as the parallel plate type, there is only one sense electrode at the sensor and the capacitance to ground is measured. If the object to be sensed is generally conductive and grounded it can effectively form the second electrode such that movement of the object towards or away from the primary sense electrode changes the capacitance and this change is measured and related to the distance or proximity of the object.
If the object to be sensed is instead not electrically conducting, a second stationary electrode is incorporated at a fixed distance away and connected to ground and the object to be sensed is passed between the two electrodes giving rise to a change in capacitance. In the second case, called the fringe field type, there are instead two sense electrodes disposed near one another at the sensor and the object which is sensed changes the capacitance between them by changing the electric field by dielectric or conductive effects. The resulting change in capacitance is sensed and this can then be related to a change in distance or proximity of the object. Fringe field type capacitive proximity sensors are widely used industrially in manufacturing applications where sensor installations are typically specified and fixed, and other potentially interfering environmental factors can be controlled.
Such devices nevertheless also frequently incorporate an additional electrode to separately sense for and thereby compensate for drift due to surface contamination. The maximum sensing distance is the sensor range and this is related to the sensitivity of the capacitance change sensing technique, the nature and size of the object to be detected and the physical size of the sense electrodes. Larger sense electrodes provide greater range.
More sensitive detection provides greater range with a given electrode size and a given object to be sensed, which is a performance advantage in applications where larger electrode structures are undesirable and greater range is desired. However, more sensitive detection of changes in capacitance does not by itself provide reliability where significant capacitance changes can also arise due to environmental factors.
The present invention has been developed to overcome the shortcomings of the hereinbefore known systems in order to provide for a capacitive sensor system with increased sensitivity and reliability.
This is accomplished by providing a sensitive means for instead detecting the time rate of change of capacitance only. This quantity is denoted mathematically as dC/dt and is distinctly different from a measurement of the difference between two capacitances as is typical of the prior art.             ⅆ      C              ⅆ      t        =      the    ⁢          xe2x80x83        ⁢    rate    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    change    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    capacitance    ⁢          xe2x80x83        ⁢    with    ⁢          xe2x80x83        ⁢    respect    ⁢          xe2x80x83        ⁢    to    ⁢          xe2x80x83        ⁢    time  
This is hence contrary to the known prior art systems where instead detection is based on a change in capacitance.
In the present invention, the detection, which is performed in the phase domain, utilizing a continuously operating control loop, is thereby advantageously insensitive to gradual changes in capacitance due to changes in the environment which may be of any absolute magnitude provided that these changes occur over sufficient lengths of time and hence at rates which are below the detection sensitivity for dC/dt.
It should be appreciated that although a time rate of change signal could in principle alternatively be derived from the output of various prior art capacitive sensors, which instead measure capacitance change, by electronically differentiating that signal, such a derived signal would not then provide the required reliable and sensitive detection. This is because the very act of differentiating a sensor signal makes the resulting signal noisier and hence less reliable.
In the present invention an intrinsically motion sensitive capacitive sensing means is thus provided for detecting the movement of an object, such as for example a person""s hand, in a region which is within a prescribed distance range from the sensor. The system in accordance with the present invention provides a means for reliably detecting small motions of a hand towards the sensor when it is within a sensing region. Moreover this reliability is independent of whether or not the person is electrically grounded or even intermittently grounded during sensor operation as may occur in the case of someone who is washing their hands.
This reliability inherently provides immunity to false activation because the sensor continuously adapts to the electrical characteristics of the surroundings and gradual changes in those surroundings of an overall magnitude greater than that due to the introduction of a hand into the sensing region. The sensor thereby has zero drift.
Thus, the present invention is functional in a range of different surroundings without requiring manual adjustment. In addition, the present invention is highly immune to RF and other externally generated electrical field interferences, has low electromagnetic emissions itself and consumes little power. This last mentioned feature enables extended operation by battery.
A capacitive sensor system in accordance with the present invention for controlling operation of a device in response to the rate of change in capacitance due to the motion of a proximate object generally comprises at least two sense electrodes disposed in a spaced apart relationship for enabling the establishment of an electrical field between the sense electrodes. An electronic circuit provides a control output signal in response to a rate of change in capacitance of the sense electrodes due to motion of the proximate object within the field without intermediate electronic differentiation of signals related to a change in capacitance.
Preferably, the sense electrodes are disposed on a planar surface, and in that configuration, enabling the establishment of an electric field extending outwardly and between the sense electrodes.
More particularly, the electronic circuit may include a phase locked frequency control loop (PLL) which includes a voltage controlled oscillator (VCO), a fixed frequency reference oscillator for the VCO to follow, a phase/frequency comparator, a phase delay network for delaying the phase of the VCO output with respect to that of the reference and which acts to cause the VCO frequency to run ahead of the reference oscillator when the loop is phase locked and a loop filter which integrates the phase error signal from the comparator and thereby defines the dynamic response of the loop.
The characteristics of the loop filter are such as to slow down and in fact match the dynamic response of the loop to the characteristic timescale of motion of the object to be detected. Additionally, a phase sensitive trigger circuit is connected between the VCO and the reference oscillator and generates the sensor output signal whenever those two signals are in phase.
The VCO is connected to the sense electrodes such that any increases in capacitance there act to slow the VCO frequency and vice versa. A capacitance change caused by an object moving into the sensing region of the sense electrodes causes a phase shift in the operating frequency with respect to that of the reference which is greater for greater rates of change in capacitance of the sense electrodes.
The phase error signal thus generated by the comparator is integrated in the loop filter and if the phase error is accumulated at a fast enough rate, such that the phase shift exceeds the threshold defined by the phase delay network, then a sensor output or trigger signal is generated. This signal can then be used for control of another device, such as a soap pump where the sensor is used to detect hand motion near a non-contact soap dispenser, or a proximity indication via connection to a display or alarm device.
In the preferred embodiment of the present invention the trigger signal generating circuit includes a D-flop circuit and in an alternative embodiment the trigger circuit includes a voltage comparator.
In the preferred embodiment a frequency divider is included between the VCO and the phase/frequency comparator which causes the VCO frequency to operate at a frequency which is a fixed multiple of that of the reference oscillator.
Also, in the preferred embodiment of the present invention the control circuit incorporates an additional feedback path for the loop which is parallel to the loop filter and is for eliminating multiple trigger signals for very large phase delay errors which would otherwise be caused by very large dC/dt""s generated at the sense electrodes. This feedback path incorporates a circuit which is adaptive to very large phase error signals in such a way that for small error signals it provides negligible output while for very large signals it does not allow the phase difference to move out of the +/xe2x88x9290 degree range. In the preferred embodiment this feedback path incorporates diodes in conjunction with an RC demodulating network.
More particularly, the VCO provides an operating frequency to the sense electrodes which is sufficiently high to insure that if the object is a human hand and the individual is grounded that the hand is nevertheless detected as a dielectric object. This eliminates any possible detection artifacts due to variations in the electrical groundedness of the hand and, as is known in the prior art, places a minimum operating frequency requirement on the sensor of a few hundred kilohertz. Hence, in the preferred embodiment and where the object is a hand and the device is a soap dispenser, the operating frequency is set to about 0.5 MHz. In this regard and alternatively for other sensing applications, the avoidance of conduction effects may impose other preferred constraints on the operating frequency. Such constraints are within the scope of the invention.
Preferably a grounded shield electrode is also provided and disposed in a spaced apart relationship parallel to and above the sense electrodes. This eliminates the sensing electric field in the region above the shield in order that that region may be utilized without falsely activating the system. In an alternative embodiment the shield electrode may be split into two halves and each half driven at the same voltage as the opposing sense electrode so as to reduce the capacitance between the sense electrodes and shield and thereby enhance the sensitivity and hence sensor range. This alternative requires additional electronic circuitry to generate the voltage waveform for the shield.